Feather-like structures in oxidized ultrananocrystalline diamond films

Ultrananocrystalline diamond films are conventionally described as diamond nanograins embedded in a sp2-rich amorphous carbon matrix. It was recently reported that as-deposited ultrananocrystalline diamond films consisted of dendrite-/nanofeather-like diamond aggregates embedded in a sp2-rich phase across the thickness. Here, we reveal that controlled oxidation at 725 °C selectively removes the intergranular sp2 phase, exposing the hierarchical architecture throughout the film thickness. Cross-sectional scanning electron microscopy and depth-resolved Raman spectroscopy support that this morphology originates during growth via intense secondary nucleation and constrained lateral crystallite expansion. These results challenge the grain matrix model and provide a scalable route to anisotropic, high-surface-area diamond coatings.